Mortgage Calculator: How It Works
A mortgage calculator answers one question — what will this house cost me every month — but the honest answer includes far more than principal and interest. This page covers the payment itself, the deposit that sets it, and the running costs that turn a comfortable payment into a stretched one.
The four parts of a housing payment
Lenders and budgeting guides often use the shorthand PITI:
- Principal — the slice of each payment that reduces the debt.
- Interest — the lender's charge on the balance still owed.
- Taxes — property or council taxes, often collected monthly into an escrow account.
- Insurance — buildings insurance, plus mortgage insurance where the deposit is small.
Only the first two come out of the loan formula. The other two vary by location and can add 15–30% on top of the figure a basic calculator shows, which is why a payment that looked affordable at the offer stage can feel tight at completion.
Deposit, loan-to-value and why 20% is the magic number
Loan-to-value (LTV) is the loan divided by the property price. A £400,000 home with an £80,000 deposit is an 80% LTV. LTV drives two things at once: the interest rate you are offered, and whether you are charged mortgage insurance.
| Deposit | LTV | Typical effect |
|---|---|---|
| 5% | 95% | Highest rates; mortgage insurance almost always required |
| 10% | 90% | Noticeably better rates; insurance still common |
| 15% | 85% | Mainstream pricing |
| 20% | 80% | Insurance normally drops away; best widely available tier |
| 40% | 60% | Lowest advertised rates |
Because the tiers are steps rather than a smooth curve, being £2,000 short of the next band can cost more over the term than the £2,000 itself. If you are close to a threshold, it is worth checking whether a slightly smaller purchase price or a slightly larger deposit tips you over.
A worked example
A £400,000 property, £80,000 deposit, £320,000 borrowed at 4.5% over 25 years:
| Monthly principal & interest | £1,778 |
|---|---|
| Total repaid over 25 years | £533,400 |
| Total interest | £213,400 |
| Interest in year 1 alone | £14,270 |
| Principal repaid in year 1 | £7,066 |
After twelve payments totalling £21,336 the debt has fallen by about £7,000. That is not a fault in the loan — it is simply what charging interest on a large balance looks like.
Term length: the trade-off in numbers
On the same £320,000 at 4.5%:
| Term | Monthly | Total interest |
|---|---|---|
| 15 years | £2,448 | £120,640 |
| 20 years | £2,024 | £165,760 |
| 25 years | £1,778 | £213,400 |
| 30 years | £1,621 | £263,560 |
The 30-year option saves £157 a month against the 25-year and costs £50,160 more overall. A useful middle path many lenders allow: take the longer term for safety, then overpay voluntarily. You get the low contractual payment as a floor and the short-term economics whenever you can afford them.
Fixed, tracker and the reset you should plan for
A fixed rate locks your payment for an initial period — commonly two to five years — after which the loan usually reverts to a much higher standard variable rate. The single most expensive mistake in mortgage borrowing is drifting onto that reversion rate. Set a reminder for six months before your fix ends and check the payment at a rate two or three percentage points higher than today's, so you know what a bad reset would look like before it happens.
Affordability rules of thumb
Lenders typically cap borrowing around 4 to 4.5 times gross annual income and stress-test your ability to pay at a rate well above the one you are offered. A common household guideline is keeping total housing costs — PITI, not just the loan — under about a third of gross income, and all debt payments under about 40%. These are guidelines, not laws; your own fixed commitments matter more than any ratio.